1. Field of the Invention
The invention relates generally to cellulose-based articles and a method to improve properties of the cellulose-based articles, including the water resistance, oil and grease resistance, wet and dry strength, or softness of the articles.
2. Background Art
Cellulose-based compositions are used in a wide range of products, and can include general categories such as paper and paper-board. Specific end use products range from sanitary napkins, cardboard boxes, paper (writing, copying, photographic, etc.), wet wipes, paper plates, food containers, and many others. Many of these products also include folds or bends, such as compartments in a paper plate or food container, creating additional manufacturing concerns.
Cellulose-based compositions are often modified for end-use applications. Various chemicals added to these cellulose-based compositions can improve desired properties, such as wet and dry strength, softness, water resistance, oil and grease resistance, and others. Unfortunately, however, when steps are taken to increase one property of the product, other characteristics of the product are often adversely affected.
As one example of modifying a cellulose-based composition, in the area of oil and grease resistance, there are many packages, such as pizza boxes and hamburger wrappers, which must be treated to prevent the unsightly staining of the package by the oil and grease from the food or other items that are packaged. Current treatments used for oil and grease resistance include treatment with fluorocarbons or extrusion coating the paper with a layer of polymer, such as LDPE. Fluorocarbon treatment often causes issues with consumer perception; LDPE coating often requires a high coating thickness, increasing costs.
As another example, water resistance/barrier is another important attribute needed in many paper and board applications, including corrugated boxes for cool storage of fruits and vegetables, as well as fish and meat packaging. Wax coatings are often used to provide the needed water resistance. These wax coatings are typically costly due to the high coating thickness required. The wax coatings also cause problems as the waxed boxes cannot be recycled in the same way as non-waxed boxes.
As a third example of enhancing the performance of cellulose-based compositions, photographic quality paper is often based on a multilayer design which consists of a paper substrate with a water impermeable polymer layer. This is often further coated with an overcoat of a water absorbent layer, and optionally an ink-receptive top layer (often containing cationic functionality to bind with pigments).
The above examples illustrate coating a cellulose-based composition with a polymer or other chemical after forming the paper or board. A polymer coating can be formed by processes such as spraying a polymer dispersion onto the paper, or by coextruding a polymer layer, for example. Dispersions or emulsions have also been added to an aqueous suspension containing cellulosic fibers, optional fillers and various additives. The aqueous suspension is fed into a headbox ejecting the suspension onto a wire where a wet web of paper is formed. The water drained from the wire, referred to as white water, is usually partly recirculated in the papermaking process.
Several references disclose the use of various thermoplastic dispersions, as a coating on paper and other substrates, to impart specific properties including heat sealability, water and or oil barrier, including WO2005/021638, DE10109992, and EP0972794. WO99/24492 discloses the use of certain polyolefin dispersions, specifically ethylene-styrene interpolymers, for use as a barrier coating on paper. WO98/03731 discloses the use of a dispersion of ethylene-acrylic acid copolymer (EAA) added in the wet end of the papermaking process to impart sizing (water resistance) to the finished “cellulosic article.” U.S. Pat. No. 4,775,713 discloses aqueous dispersions containing various thermoplastics and a thermoplastic polymer containing a carboxylic acid salt group.
Another important attribute for efficient operations within a paper mill is the ability to reclaim or recycle materials used in the process, such as white water recirculation and the rebroking of edge trim and paper made during startup and shutdown (transforming the paper back into a slurry of pulp). The coating of the cellulosic fibers after forming a web of paper, or paper-board can have negative effects on the rebrokeability of the paper. Dispersions added to the process prior to forming the paper can negatively affect white water recirculation.
Accordingly, there exists a need for determining dispersion compositions useful as a paper coating or additive to enhance specific performance attributes. There also exists a need to determine a narrower range of dispersion compositions which can enhance specific performance attributes while not adversely affecting other attributes, such as improving strength while maintaining softness, for example. Further, there exists a need to determine methods and compositions which allow the recycling and reclamation of process materials to improve the manufacturing efficiency and cost of the papermaking process.